This application claims the benefit of Japanese Patent Application No. Hei. 10-171438, filed in Japan on Jun. 18, 1998, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a gas sensor, such as an oxygen sensor, HC sensor, or NOx sensor, for detecting a component in gas to be measured,
2. Description of the Related Art Conventional gas sensors have a structure in which a metallic casing accommodates a bar-like or cylindrical sensing element which has a sensing portion formed at its tip end and is adapted to detect a component in gas. The metallic casing includes a combination of a plurality of cylindrical members, such as a metallic shell, a protector, an inner cylindrical member, and an outer cylindrical member. The metallic shell has a screw portion formed on its outer surface that is used for attachment. The protector is connected to the metallic shell in such a manner as to cover the sensing portion of a sensing element which projects from one end of the metallic shell. The inner cylindrical member is connected to the other end of the metallic shell and adapted to cover the sensing element which extends rearward from the metallic shell; i.e., opposite the protector with respect to the metallic shell. The outer cylindrical member is connected to a rear end portion of the inner cylindrical member and allows a lead wire from the sensing element to extend rearward from a rear open end thereof.
The inner cylindrical member and the outer cylindrical member are made of, for example, stainless steel, In many gas sensors, the inner and outer cylindrical members are caulked together. Specifically, an end portion of the outer cylindrical member is fitted onto a corresponding end portion of the inner cylindrical member to thereby form an overlap zone. In the overlap zone, the outer cylindrical member is circumferentially caulked to the inner cylindrical member, thereby forming an annular caulked portion and thus bonding the members in an airtight manner.
Adjusting the hardness of the inner and outer cylindrical members before the inner and outer cylindrical members are caulked as described above is an important measure for attaining a caulked seal which is free from play or loosening and exhibits excellent airtightness. For example, Japanese Patent Application Laid-Open (kokai) No. 9-210953 describes adjustment of the hardness of a stainless steel inner cylindrical member (inner cover) of an oxygen sensor to Hv 150 to 350 on the Vickers scale and the hardness of a stainless steel outer cylindrical member (outer cover) of the oxygen sensor to Hv 100 to 300 on the Vickers scale in order to attain a tightly caulked seal having excellent resistance to vibration. The publication also describes an embodiment in which the outer cylindrical member having a thickness of 0.5 mm a hardness of Hv 150 and the inner cylindrical member having a thickness of 0.7 mm has a hardness of Hv 240.
Gas sensors, e.g., oxygen sensors are often mounted on an exhaust pipe or exhaust manifold through which high-temperature exhaust gas flows, Accordingly, sensor temperature becomes approximately equal to ambient temperature in an idle state, whereas it increases to hundreds of degrees centigrade in high-speed high-load operation. Therefore, the oxygen sensor is repeatedly subjected to a considerably severe thermal shock. The publication described above discusses the effects of the caulked seal being subjected to vibration or water splashes (or submergence), but fails to address in detail the effects of thermal shock. The inventors of the present invention carried out intensive studies, and as a result, found that when the outer and inner cylindrical members assume the values of hardness appearing in the publication, in some cases the caulked portion fails to maintain good airtightness after being subjected to a severe thermal history.
An object of the present invention is to provide a gas sensor having a structure such that an inner cylindrical member and an outer cylindrical member, which cover a sensing element, are caulked together to form a caulked portion, and the caulked portion can maintain good airtightness even in a working environment involving, for example, thermal shocks.
To achieve the above object, the present invention provides a gas sensor comprising a bar-like or cylindrical sensing element and a cylindrical casing. The sensing element has a sensing portion formed at a tip end portion thereof and is adapted to detect a component in gas to be measured (gas under measurement).
The cylindrical casing covers the sensing element while the gas under measurement is permitted to flow therethrough to the sensing portion. The casing includes at least two axially adjacent cylindrical members. An end portion of one cylindrical member (hereinafter referred to as an inner member) is disposed within a corresponding end portion of the other cylindrical member (hereinafter referred to as an outer member) to thereby form an overlap zone.
The inner cylindrical member has a Vickers hardness H1 of Hv 250 to 430; the outer cylindrical member has a Vickers hardness H2 of Hv 160 to 330; and the hardness difference xe2x80x9cH1-H2xe2x80x9d therebetween is not less than 30. In the overlap zone, the inner cylindrical member and the outer cylindrical member are circumferentially caulked in an airtight manner.
According to the above-described structure of the gas sensor,the inner cylindrical member and the outer cylindrical member assume hardness values within the above ranges, so that the caulked portion can maintain good airtightness even in a working environment involving, for example, thermal shocks.
A key structural point of the present invention is that the Vickers hardness H2 of the outer cylindrical member assumes a value of at least Hv 160, which is higher than that disclosed in the aforementioned publication. Through increase in the Vickers hardness H2 of the outer cylindrical member, the hardness of the caulked portion is increased, thereby suppressing deformation of the caulked portion caused by thermal stress induced therein during exposure to heat cycles, and thus preventing the caulked portion from loosening, while yielding a resultant improvement in airtightness. Further, the Vickers hardness H1 of the inner cylindrical member is adjusted to not less than Hv 250, and the hardness difference xe2x80x9cH1-H2xe2x80x9d between the inner and outer cylindrical members is set to not less than 30. Thus, when the outer cylindrical member having the above high hardness is caulked onto the inner cylindrical member, the inner cylindrical member can sufficiently receive the caulking force, whereby the caulked portion can attain strong adhesion.
When the Vickers hardness H2 of the outer cylindrical member is less than Hv 160, the thermal shock resistance of the caulked portion deteriorates, resulting in a failure to secure sufficient airtightness upon exposure to repeated heat cycles. When the hardness difference xe2x80x9cH1-H2xe2x80x9d between the inner and outer cylindrical members is less than 30, the inner cylindrical member fails to sufficiently receive the caulking force and thus suffers an undesirably intensive deformation, Also, the caulking-related deformation of the outer cylindrical member becomes unsatisfactory. As a result, the caulked portion potentially fails to maintain airtightness. When the Vickers hardness H1 of the inner cylindrical member is less than Hv 250 or the Vickers hardness H2 of the outer cylindrical member is in excess of Hv 330, the thermal shock resistance of the caulked portion deteriorates and the caulked portion fails to maintain sufficient airtightness upon exposure to repeated heat cycles. The reason for this is as follows. When the Vickers hardness H1 of the inner cylindrical member is less than Hv 250, rigidity of the inner cylindrical member becomes insufficient; consequently, conceivably, the inner cylindrical member undergoes undesirably intensive deformation during caulking, resulting in impairment in airtightness. When the Vickers hardness H2 of the outer cylindrical member is in excess of Hv 330, the rigidity of the outer cylindrical member becomes too high; consequently, conceivably, the deformation of the outer cylindrical member becomes unsatisfactory during caulking, or forcible caulking may cause nonuniform deformation or cracking, resulting in impairment in airtightness. Notably, the hardness difference xe2x80x9cH1-H2xe2x80x9d is more preferably not less than 50.
Preferably, the thickness of the inner cylindrical member is 0.6 mm to 1.0 mm, and the thickness of the outer cylindrical member is 0.2 mm to 0.6 mm. When the inner cylindrical member has a thickness of less than 0.6 mm, even when the hardness H1 of the inner cylindrical member is not less than Hv 250, the inner cylindrical member fails to sufficiently receive the caulking force, potentially resulting in impairment in airtightness of the caulked portion. When the thickness of the inner cylindrical member is in excess of 1.0 mm, caulking may become difficult to perform. When the thickness of the outer cylindrical member is less than 0.2 mm, the outer cylindrical member is apt to deform in association with a lack of strength thereof. When the thickness of the outer cylindrical member is in excess of 0.6 mm, the caulking force fails to be sufficiently exerted on the inner cylindrical member, and thus a sound caulked portion becomes difficult to obtain.
According to the studies conducted by the present inventors, so long as the hardness difference between the inner and outer cylindrical members is not less than 30 (preferably, not less than 50), through employment of a hardness H2 of the outer cylindrical member of at least Hv 160, the caulked portion maintains good thermal shock resistance at a hardness H1 of the internal cylindrical member of up to Hv 430. This indicates that increasing the hardness of the inner cylindrical member results in an expansion of the hardness range of the outer cylindrical member over which a strong caulked portion can be attained. In other words, even when hardness varies among outer cylindrical members due to process-related causes, a strong caulked portion having excellent thermal shock resistance can be readily obtained. Specifically, employment of a hardness H1 of the inner cylindrical member of not less than Hv 300 has the effect of markedly expanding the hardness range of the outer cylindrical member over which a strong caulked portion can be formed, thereby facilitating process control. Accordingly, more preferably, the hardness H1 of the inner cylindrical member assumes a value of not less than Hv 330. The inner and outer cylindrical members may be made of an iron material that can be cold-worked, such as an austenitic steel. The hardness of such material is generally not greater than Hv 430.
When the outer cylindrical member is made of an austenitic stainless steel through cold-working, such as forging, drawing, or deep-drawing, the Vickers hardness H2 thereof is more preferably adjusted to Hv 175 to 275. This is done for the following reason.
Advancement of work hardening often causes a cold-worked member to assume a high hardness of Hv 350 to 400, which is unsuitable for caulking. Before being caulked, the member is desirably annealed at an appropriate temperature so that the hardness thereof falls within a range adequate for caulking. In this case, the hardness of the cold-worked member after annealing does not necessarily vary linearly with annealing temperature. Specifically, the hardness varies with temperature to a relatively small extent until temperature rises to a certain temperature (first temperature region). When the certain temperature is reached, the hardness decreases abruptly within a certain relatively narrow temperature region (second temperature region). In a temperature region (third temperature region) higher than the second temperature region, the hardness again varies with temperature to a relatively small extent.
The Vickers hardness H2 of the outer cylindrical member can be adjusted to the aforementioned range through annealing in the third temperature region or at the high-temperature side of the second temperature region, in which the hardness varies relatively slightly. Annealing within such a temperature region stably imparts to the outer cylindrical member a hardness adequate for caulking even when the annealing temperature varies to some extent. The inner cylindrical member may be adjusted to a hardness of Hv 250 to 430 without undergoing annealing after being cold-worked. Also, through employment of warm working, the outer cylindrical member may be adjusted to a hardness of Hv 160 to 330 without undergoing annealing after being cold-worked,
In terms of corrosion resistance, the inner cylindrical member and the outer cylindrical member are preferably made of stainless steel. Examples of such stainless steel include stainless steel as described in JIS G4304; specifically, austenitic stainless steel (stainless steel which shows an austenitic structure at room temperature) such as SUS201; SUS202, SUS301, SUS301J, SUS302, SUS302B, SUS304, SUS304L, SUS304N1, SUS304N2, SUS304LN, SUS305, SUS309S, SUS310S, SUS316, SUS316L, SUS316N, SUS316LN, SUS316J1, SUS316J1L, SUS317, SUS317L, SUS317J1, SUS321, SUS347, or SUSXM15J1; austenitic-ferritic stainless steel (stainless steel which shows a two-phase structure of austenite and ferrite) such as SUS329J1 or SUS329J2L; ferritic stainless steel (stainless steel which, upon undergoing heat treatment, does not harden and shows a ferritic structure) such as SUS405, SUS410L, SUS429, SUS430, SUS430LX, SUS434, SUS436L, SUS444, SUS447J1, or SUSXM27; and precipitation hardening stainless steel (stainless steel which, through addition of, for example, aluminum and copper, can be hardened through precipitation of a compound primarily formed of the elements which is effected by heat treatment) such as SUS631.
The concept of xe2x80x9cstainless steelxe2x80x9d appearing in the present invention includes the following beat-resisting steel.
(1) Austenitic Heat-resisting Steel
Examples of austenitic heat-resisting steel include steel of a composition specified in, for example, JIS G4311 or G4312; specifically, SUS31, SUH35, SUH36, SUH37, SUH38, SUH309, SUH310, SUH330, SUH660, or SUH661. In the present invention, austenitic heat-resisting steel is included in the concept of the austenitic stainless steel.
(2) Ferritic Heat-resisting Steel
Examples of ferritic heat-resisting steel include steel of a composition specified in, for example, JIS G4311 or G4312; specifically, SUH446. In the present invention, the ferritic heat-resisting steel is included in the concept of the ferritic stainless steel.
(3) Martensitic Heat-resisting Steel
Examples of martensitic heat-resisting steel include steel of a composition specified in, for example, JIS G4311 or G4312; specifically, SUS1, SUS3, SUS4, SUS11, SUS600, or SUS616. In the present invention, the martensitic heat-resisting steel is included in the concept of the martensitic stainless steel.
In terms of workability (particularly cold workability) and hence improvement in manufacturing efficiency for members, ferritic stainless steel and austenitic stainless steel are preferred. In view of the gas sensor being used at high temperature and high humidity, austenitic stainless steel is particularly preferred.
In the overlap zone between the inner cylindrical member and the outer cylindrical member, the outer cylindrical member may be caulked to the inner cylindrical member so as to form an annular main caulked portion along a circumferential direction and a rotation-prevention portion. The rotation-prevention portion is adapted to prevent relative rotation between the inner and outer cylindrical members about their common axis at the main caulked portion. The main caulked portion includes a cylindrical contact surface between the inner and outer cylindrical members, thereby providing excellent airtightness and thus reliably preventing, for example, the entry of water into the interior of the inner cylindrical member. Through formation of the rotation-prevention portion, even when a torsional force about the common axis of the inner and outer cylindrical members is exerted between the inner and outer cylindrical members, relative rotation between the inner and outer cylindrical members is prevented, thereby further reliably establishing airtightness at the caulked portion.
The rotation-prevention portion may assume the form of an auxiliary caulked portion which is located on at least one side of the main caulked portion as viewed in the axial direction of the outer cylindrical member and is formed through caulking of the outer cylindrical member to the inner cylindrical member, The auxiliary caulked portion can be readily formed and yields an excellent rotation-prevention effect. Specifically, the auxiliary caulked portion is located adjacent to and spaced a predetermined distance from the main caulked portion in the axial direction of the outer cylindrical member, and may assume an annular form along the circumference of the outer cylindrical member. Through formation of the annular auxiliary caulked portion adjacent to the main caulked portion, the rotation-prevention effect can be further improved. Further, the auxiliary caulked portion may assume a polygonal transverse cross section. Since the contact faces between the inner and outer cylindrical members define a prismatic form, relative rotation between the inner and outer cylindrical members is prevented even under application of a torsional force.
The auxiliary caulked portion is preferably located closer to the sensing clement than is the main caulked portion. Since a tip portion of the gas sensor is often exposed to high temperature, the main caulked portion, which is primarily intended to establish airtightness, is more preferably located away from such a heat source through employment of the above-described arrangement.
The main caulked portion and the auxiliary caulked portion can be simultaneously formed through use of two caulking punch units which each include a plurality of caulking punches for compressing the outer cylindrical member circumferentially from outside and which are spaced a predetermined distance apart in the axial direction of the outer cylindrical member. Since the main caulked portion and the auxiliary caulked portion are simultaneously formed in a single caulking step, this process is not only efficient but also yields the following effect. Through compression by the caulking punches, the outer cylindrical member locally bites and is pressed against the inner cylindrical member, thereby forming the caulked portion. The biting deformation is apt to be accompanied by a wrinkling portion or a lifting portion which is formed on the outer cylindrical member along the pressed portion. If the main caulked portion and the auxiliary caulked portion are formed sequentially, the first caulked portion is affected by a wrinkling portion or a lifting portion associated with the caulked portion formed next, potentially impairing airtightness. However, through simultaneous formation of the main and auxiliary caulked portions as described above, a wrinkling portion or a lifting portion can be confined to within a region located between the main and auxiliary caulked portions, thereby establishing sufficient adhesion or airtightness at both main and auxiliary caulked portions.
In the above-described gas sensor structure, the sensing element may be formed such that the sensing portion thereof includes an oxygen concentration cell element which operates by using as reference gas air introduced into the casing. In this case, the inner cylindrical member serves as a main barrel for accommodating the sensing element, and the outer cylindrical member serves as a cylindrical filter assembly independent of the main barrel. The main barrel and the cylindrical filter assembly are disposed substantially coaxially. The filter assembly is connected to a rear portion of the main barrel while permitting a lead wire from the sensing element to extend rearward of the filter assembly. The filter assembly may include a filter holder and a filter. The filter holder assumes a cylindrical form and is connected to a rear portion of the main barrel substantially coaxially while the interior thereof communicates with the interior of the main barrel. The filter holder has at least one gas inlet hole formed in a wall thereof. The filter is disposed in such a manner as to cover the gas inlet hole(s) formed in the filter holder and functions to permit transmission of gas while blocking transmission of liquid. In this case, air is introduced into the main barrel through the filter and the gas inlet hole(s). Herein, with respect to the axial direction of the oxygen sensing element, the term xe2x80x9cfrontxe2x80x9d implies a portion toward a tip end of the oxygen sensing element, and the term xe2x80x9crearxe2x80x9d implies a portion away from the tip end.
Tie above-described structure is characterized in that an air-permeable structure including the filter assumes the form of the filter assembly which is independent of the main barrel and is integrally connected to the main barrel, thereby yielding the following effects.
(1) The filter assembly can be assembled independently of, for example, attachment of the oxygen sensing element into the main barrel. Thus, the filter assembly can be assembled quite efficiently without interference caused by, for example, a lead wire of the sensing element.
(2) Since installation of component members into the main barrel and assembling of the filter assembly can be carried out in parallel, productivity is improved drastically. Even when the filter assembly suffers a defect in installation of the filter, if the defect is discovered before the filter assembly is coupled to the main barrel, the defect can be prevented from affecting a gas sensor product, thereby avoiding potential waste of component members.
The filter used in the present invention may be a waterproof, air-permeable, porous resin filter made of fluoroplastic, such as polytetrafluoroethylene. Specifically, such a material for the filter may have a porous fibrous structure (described in, for example, Japanese Patent Publication (kokoku) Nos. 42-13560, 51-18991, 56-45773, and 56-17216, and Japanese Patent Application Laid-Open (kokai) Nos. 58-145735, 59-152825, 3-221541, 7-126428, and 7-196831; for example, Gore-Tex product of Japan Gore-Tex)) obtained by stretching an unfired compact of, for example, polytetrafluoroethylene (hereinafter called PTFE) in at least one axial direction at a heating temperature lower than the melting point of PTFE.
The filter assembly may include a filter holder, a filter, and an auxiliary filter holder. The filter holder is provided coaxially and integrally with the main barrel such that the interior of the filter holder communicates with the interior of a rear portion of the main barrel. The filter holder has at least one gas inlet hole formed in a wall thereof. The filter is disposed around the filter holder in such a manner as to cover the gas inlet hole(s) formed in the filter holder and functions to permit transmission of gas while blocking transmission of liquid. The auxiliary filter holder is formed into a cylindrical shape and is disposed around the filter. The auxiliary filter holder has at least one auxiliary gas inlet hole formed therein and holds the filter in cooperation with the filter holder. In this case, air is introduced into the main barrel through the auxiliary gas inlet hole(s), the filter, and then the gas inlet hole(s). The filter is reliably held between the inner filter holder and the outer auxiliary filter holder and can be easily attached onto the filter holder. For example, when the filter is formed into a cylindrical shape, the filter is fitted onto the filter holder, and then the auxiliary filter holder is fitted onto the filter. Subsequently, a holder coupling portion for coupling the filter holder and the auxiliary filter holder is formed at such a position as not to interfere with the gas inlet hole(s) and the auxiliary gas inlet hole(s).
A plurality of gas inlet holes and auxiliary gas inlet holes may be formed in the filter holder and the auxiliary filter holder, respectively, in such a manner as to be located at axially intermediate portions thereof and be arranged at circumferentially predetermined intervals and at mutually corresponding positions. This arrangement enables air to be uniformly introduced into the main barrel through the filter assembly. A filter of, for example, a cylindrical shape may be disposed around the filter holder, Then, while the filter is held between the filter holder and the auxiliary filter holder, the auxiliary filter holder may be caulked to the filter holder, thereby forming an annular filter-caulking portion along the circumference thereof. The filter-caulking portion serves as the holder coupling portion. Through employment of the filter-caulking portion as the holder coupling portion, assembling work for the filter assembly is further facilitated. The annular filter-caulking portion may be formed on the axially opposite sides of the row of gas inlet holes and auxiliary gas inlet holes, In this case, the filter-caulking portions are formed in such a manner as to hold the corresponding filter edges between the auxiliary filter holder and the filter holder. This minimizes the possibility of formation of a passage which extends from the auxiliary gas inlet holes to the gas inlet holes of the filter holder while bypassing the filter edge, and hence minimizes the possibility of water leaking into the interior of the filter holder and then into the interior of the main barrel through such a potential passage.
When the filter is formed into a cylindrical shape and is disposed around the filter holder as described above, the auxiliary filter holder can be disposed such that the axially rear end thereof is aligned with that of the filter. Thus, the filter-caulking portion can be circumferentially formed along the rear end of the filter. This arrangement enables a worker to visually observe the rear end face of the filter located between the filter holder and the auxiliary filter holder. When the cylindrical filter is fitted onto the filter holder, and then the auxiliary filter holder is axially moved so as to be fitted onto the filter, the filter may move together with the auxiliary filter holder with a resultant dislocation from an intended position. If the filter-caulking portion is formed without correction of the dislocation, the filter is dislocated from the filter-caulking portion, resulting in incomplete seal. However, since a worker can visually observe the rear end face of the filter as described above, any such defect in filter caulking can be easily discovered and corrected accordingly.
When the filter-caulking portion is formed at the front end portion of the auxiliary filter holder, a filter inspection window may be formed in the filter-caulking portion so as to partially expose the filter. Through observation of the filter through the window, a worker can easily judge whether the filter is properly caulked by the filter-caulking portion located at the front end portion of the auxiliary filter holder.